1. Field of the Invention
This invention relates generally to communications to and from processors and more particularly to a control system for inhibiting spurious signal transmissions which might occur as a result of decreases in power supply voltage.
2. Related Applications
The control system of the present invention may be employed in conjunction with and carried on an interface board which interconnects a postage value determining system processor with a plurality of peripheral devices. Such interface is described in a copending application of Daniel F. Dlugos et al. entitled Postage Scale Peripheral Interface, Ser. No. 235,241, filed Feb. 17, 1981 and assigned to the assignee of the present invention. The interface includes a peripheral microcomputer, the transmit and receive lines of which are multiplexed among selected mailing system peripherals. Upon receipt of an appropriate command from a postage scale system processor, the peripheral microcomputer establishes communications links with selected peripheral devices, receiving data from either the system processor or a peripheral and transmitting such data employing appropriate communications subroutines.
Among the mailing system peripheral devices available are one or more electronic postage meters which set postage amounts. The electronic postage meters are coupled to a mailing machine which includes a rotating drum mechanism for printing the postage amount set by the meter. The mailing machine is controlled by an optically coupled relay having a light source and a photodetector.
Unfortunately, during mailing system operating conditions, the communications links between the interface microcomputer and the peripherals were subject to spurious signal transmissions at transient power stages such as power up and power down.
Spurious signal transmissions occurred in instances wherein the peripheral devices such as a mailing machine utilized a separate power supply. As the interface power went off, the supply voltage passed through a transient state wherein the logic controlling the communications lines lost functional control of the signal levels on the lines. During this stage power might still be applied to a peripheral. With respect to the mailing machines, the optically coupled relay interpreted the decaying voltage level on its "trip" line at a trip signal and actuated the mailing machine to print the postage amount previously set by the meter.
Other problems were also encountered during transient power states. Some peripheral devices which were still capable of reading the communication lines interpreted changes in signal levels as request or data transmission signals.
While some of the difficulties could be alleviated somewhat by simultaneously applying power to both the peripherals and the communications controller interface so both systems would initialize, this was often impractical and ineffective with respect to optically coupled relays and power down conditions.
A further problem was that a momentary dropout in line supply voltage may result in a drop in regulated power supply voltage to the communications processor to a level which would result in a momentary loss of control. When the voltage level was restored, the processor might commence random program execution and might even commence a meter tripping cycle.